Three pathways have been identified by which single-stranded circular phage DNAs are converted to duplex DNA in reactions catalyzed by purified proteins isolated from uninfected E. coli: (1) DNA synthesis of fd or M13 DNA requires RNA polymerase, DNA binding protein, DNA elongation factors I and III, dnaZ gene product, and DNA polymerase III; (2) Synthesis of ST-1 or G4 DNA requires dnaG and dnaZ gene products, DNA binding protein, DNA elongation factors I and III and DNA polymerase III; and, (3) Synthesis of phi X174 DNA requires dnaB, dnaC, dnaG, and dnaZ gene products, DNA binding protein, DNA elongation factors I and III, DNA polymerase III and replication factors X, Y, and Z. The functions of these purified proteins have been studied: (1) The dnaB gene product has ribonucleoside triphosphatase activity which is stimulated by single-stranded DNA; (2) dnaB and dnaC gene products interact physically and functionally in vitro; (3) Replication factor Y has phi X174 DNA-dependent ATPase activity; and (4) dnaZ gene product and elongation factors I and III function with either DNA polymerase III or II in the elongation of single-stranded DNA primed with DNA or RNA.